Oxygen non-stoichiometry and redox thermodynamics of LaMn _1-XCo_xO_3-δ

An experimental study of the oxygen non-stoichiometry and redox thermodynamics of LaMn_1-xCo_xO_3±δ (x = 0.25 and 0.75) is presented. The system shows partial solid solubility of LaCoO₃ in LaMnO₃ and vice versa. While both samples consist of two phases at room temperature, quenching experiments show that the x = 0.25 sample is single phase at high temperatures for the whole range of study, 0.6 < - log(pO₂/atm) < 14. The x = 0.75 sample consist of two phases even at high temperatures. This does not necessarily represent a problem for applications, and both samples are studied in details. The oxygen non-stoichiometry versus oxygen partial pressure are measured at three temperatures of 1223, 1273, and 1323 K by coulometric titration and thermogravimetry, for x = 0.25 and 0.75, respectively. The extracted enthalpies of oxidation do not vary significantly with stoichiometry, and are - 680 ± 40 and -522±40kJmol O₂^-1 for x = 0.25 and 0.75, respectively. These data and analyses of literature data for LaMnO ₃ and LaCoO₃ show that the enthalpy of oxidation varies nearly linearly with x. Solid solution thermodynamic models are used to analyze the redox mechanism.